This invention relates to an apparatus for controlling an oil pressure for a vehicular automatic transmission.
As this kind of conventional apparatus, Japanese Published Unexamined Patent Application No. 255645/1987 discloses an apparatus which comprises: accumulators connected to oil passages which are communicated with hydraulic engaging elements for transmission or speed changing; a regulator valve for regulating the oil pressure to be supplied to the hydraulic engaging elements; a first pressure control valve of electromagnetic type for controlling the output oil pressure of the regulator valve by causing the oil pressure to act thereupon; a second pressure control valve of electromagnetic type for controlling the back pressure of the accumulators; wherein at the time of transmission, or speed change operation, the output oil pressure of the regulator valve and the back pressure of the accumulators are varied by an electronic control circuit via both pressure control valves, whereby shocks at the time of speed changing are absorbed or alleviated.
In the above-described apparatus, if the pressure control valve is locked, due to a failure in the solenoid thereof, sticking of the spool, or the like, into a condition in which a drain port thereof is opened whereby the output oil pressure is lowered, the oil pressure to be controlled by this pressure control valve will also be kept maintained in a low pressure. As a consequence, an adequate control can no longer be effected.